The present invention relates to machine control, and, more particularly, to a method using 3-dimensional laser measurement of the true position of a machine tool to augment the accuracy and control of a machine. The invention is especially useful in the accurate machining, inspecting, or both of a part based upon a digital definition of the part. A preferred method, apparatus, and related software provide end point control of the machine tool to place holes and other features accurately on aerospace structural detail parts.
Machine tools exhibit dimensional positioning errors that are difficult to minimize. The primary contributors to these positioning errors are: (1) expansion and contraction of the machine structure and the workpiece (i.e., the part) because of thermal changes in the factory during machining, and (2) mechanical misalignments of and between individual axes of the machine. The accuracy of the machine is often so uncertain that post-machining inspection of the dimensions of the parts must be made using an independent measuring method. Such inspection requires special tools and skilled workers as well as significant factory space. It slows the production process. Failing inspection, parts must be reworked or scrapped. Post-production inspection, rework, and scrap are the result of poor design or manufacturing processes. The method of the present invention addresses the root cause for errors and, thereby, reduces the need for post-production inspection and the costs of poor quality.
National standards and xe2x80x9cbest practicesxe2x80x9d exist for determining and correcting NC machine geometry errors. (See ANSI/ASME B89.1.12M-1985, ANSI B89.6.2-1973, AMSE B.54-1992) These xe2x80x9cbest practicesxe2x80x9d constitute the currently accepted methods for achieving machine accuracy. We will discuss the standards and xe2x80x9cbest practicesxe2x80x9d briefly.
1. Thermally Controlled Environment
The machine is held at a constant temperature, e.g., 68xc2x0 F., in an air-conditioned factory. Errors arising from temperature variations are reduced, but this method does not solve the thermal error problem entirely. Three main drawbacks are:
(i) The cost of controlling the environment is high and sometimes exceeds the cost of acquiring the machine.
(ii) Thermal effects induced by the machine itself ( e.g. motor heat from driving under load, and spindle heating due to friction) still can cause machine distortion
(iii) Mechanical misalignment of axes remains uncorrected. Mechanical alignments change over time as the machine experiences normal and abnormal wear. They are essentially unpredictable, unavoidable, and difficult to control.
2. Machine Calibration
Three-axis machines have 21 error parameters in addition to the errors introduced with the machine spindle. The errors are linearity in each axis (3), straightness in each axis (6), squareness between each axis pair (3), and pitch, yaw, and roll in and between each axis (9). Machine calibration measures some or all of these 21 error parameters, then makes physical or software adjustments to the parameters which are out of tolerance. Once each error is identified, quantified, and minimized, the combination of errors are summed using the root mean squares algorithm to gain an estimate for the machine""s overall working tolerance. Machine calibration is inadequate for two reasons. First, the method requires extensive machine downtime to measure and to adjust the error parameters. The difficulty in the measurement and adjustment is compounded by the fact that thermal variation causes dimensional changes from shift to shift and day to day. Second, because of constant readjustment of the machine, the changes mean that the final set of data is not a single xe2x80x9csnapshotxe2x80x9d of the machine errors, but are a series of snapshots each of a different parameter, at a different time, as the machine changes. The root cause of inaccuracy is not fixed, but simply is accommodated between readjustments. Production is a compromise and drift occurs in the produced parts as the machine tool changes.
3. Linear Interferometry of Each Machine Axis
The X, Y, and Z axes of a machine are each equipped with a linear interferometer as an accurate positional encoder. The method allows real-time compensation for thermal growth and shrinkage, but is inadequate for at least three reasons. First, it cannot be applied to rotary axes. Second, it does not compensate for mechanical misalignments between axes. Third, it does not address the interaction between axes as thermal changes occur.
4. Volumetric Look-up Table
This method accurately measures performance of the machine in a specified dimensional envelope. The accurate performance measurements are made using an independent, highly accurate measurement machine to determine the difference between the measured data and the commanded machine position. The collection of all such errors constitutes or can be used to generate an error map. A complete error map is used in two ways. First, the error map may be used as a look-up table to determine a simple position correction to the machine when in that vicinity. Second, polynomial equations can be calculated from the error map to interpolate error corrections over the entire measured envelope. The machine command for a position is adjusted with the polynomial equations. Look-up tables are inadequate primarily because the tables are valid for only one machine temperature. At other temperatures, the machine will be larger or smaller or have a slightly different geometry. There is no guarantee that a machine will behave isometrically and return to its original geometry as temperature changes occur. So, after a laborious data collection exercise leading to an empirical table or set of equations to adjust the position of the machine based upon its history of performance, the root cause(s) for inaccuracy will still continue to degrade the effectiveness of the error map. The error map is inherently inaccurate whenever the machine has changed. As the machine continues to wear and age, variations from the measured offsets of the original error map occur. As a result, errors in part construction may increase. Frequent recalibration is necessary to continue to have an accurate correct error map.
5. Combination of Methods
Certain combinations of these methods can be used to overcome weaknesses in the individual methods, but the net effect remains: (1) long downtime of the machine to measure its true position; (2) expensive testing; and (3) only temporary, corrective results. The root cause for the inaccuracies still remains. For instance, a combination of a thermally controlled environment with machine calibration can result in an accurate machine for a period of time. The cost of controlling the environment combined with the cost of machine downtime for checking and readjusting the machine can be expensive.
6. Thermal Compensation
The axes of the machine are equipped with thermal probes. The temperature measured by each probe is used to calculate independent from the other axes the theoretical expansion of that machine axis. The expansion factors are used to compensate the feedback to the controller, thus eliminating the expansion and contraction of the machine positioning capability. A newer but similar technique called xe2x80x9creal time error correctionxe2x80x9d also uses thermal probes, but attempts to provide a 3D xe2x80x9cerror modelxe2x80x9d of the nonlinear thermal behavior of the machine structure. The error map reflects interdependence between axes, such as buckling or warping, caused by heating. Compensation is made with a complicated algorithm that is accurate only for the tested/measured envelope of variation and, then, only as the machine remains repeatable. This error model is established by gathering actual 3D machine position and corresponding temperature data over a range of temperatures, which can require significant machine downtime. It can also be difficult to place the machine in the desired thermal status. While the purpose of this technique is to avoid the costs associated with thermal control, thermal control is required to produce the error model. Thermal compensation follows the same concept as thermal control: modify the machine movement based on actual temperature measurements.
There are two main drawbacks to the thermal compensation method. First, thermal compensation requires periodic machine downtime to calibrate the sensors and the error model. Second, thermal compensation focusing on the machine does not correct for the expansion of the part or tooling fixtures. If it were possible to eliminate all positioning errors of the machine and perfectly to adjust the machine for temperature, the part could still be made out of tolerance because of the temperature effects on the part. Thermal compensation attempts to compensate for the part expansion indirectly by compensating for the machine errors caused by temperature changes. The correlation between the machine errors and the total error, however, is only a partial solution.
In U.S. Pat. No. 4,621,926, Merry, et al. describe an interferometer system for controlling non-rectilinear movement of an object. The system uses three, one-dimensional tracking laser interferometers rigidly mounted in a tracker head to track a single retroreflector mounted on the machine tool end effector. The Merry system is difficult to retrofit to an existing control system for a machine, because its laser feedback is designed to replace the conventional machine controller.
In the system of the present invention, the laser tracker operates independently from the machine controller to provide positional feedback information to the controller in trickle-fed Media blocks. [By xe2x80x9ctrickle feedxe2x80x9d we mean that motion control information is provided (downloaded) to the machine controller a little bit at a time (in single NC Media blocks, for example) rather than as a complete program.] Our much larger working envelope (ten times larger than Merry) uniquely makes our system applicable to the manufacture and assembly of large aerospace structure, like wings, and our system design allows implementation readily on a large variety of existing machine controllers.
Merry determines the location of the retroreflector using trilateration. During set up and calibration, the machine moves in a straight line at constant speed along one independent axis for the system to establish a frame of reference for the end effector and to provide coordinate data to connect the laser interferometric position measurements with the end effector motion. Each interferometer is a one-dimensional (single axis) measurement system which generates a signal proportional to the distance of the retroreflector from the interferometer. With three output signals, the Merry control system uses trilateration to calculate the location of the end effector, compares this location with the desired location based upon a stored, predetermined path for motion of the end effector (i.e., the NC program), and actuates the tool""s motive assembly to move the end effector to the next desired location. Laser trilateration has not been adopted in industry because of its cost, instability, setup geometry requirements, and natural inaccuracy. Trilateration works best if the three interferometers are widely spaced, but the retroreflector is essentially a one-axis target. To track the target, the interferometers must be close together which introduces significant interpolation or calculation errors. Futhermore, trilateration actually requires four interferometers to determine absolute, true position.
Merry""s system replaces the standard machine controller with laser interferometric position measurement actually and directly to control of the tool. By overriding the machine controller, control of the machine might be lost, for example, if chips obscure the laser beam. For high value parts, the risk of loss of control is unacceptable. The Merry system, accordingly, has not been implemented for practical use in industry because of the problems it poses.
In a preferred embodiment of the present invention, static optical machine control (SOMaC) is able to adjust the machine media to accommodate translations, rotations, or both of the machine, part, or both. SOMaC does so by measuring the position of the part and the machine and scaling for changes from the original reference location and orientation of the part and machine. SOMaC also can adjust (scale) the machine media to accommodate changes in the part, machine, or both arising from changes in factory temperature, temperature of the part, temperature of the machine, and other physical changes in the factory environment.
The SOMaC system of the present invention provides fail-safe machine control because it continues to use the machine tool""s conventional encoders, but augments the true position accuracy in static operation by providing xe2x80x9con-the-flyxe2x80x9d inspection feedback through optical measurement of the true position. Our system corrects for the machine positioning errors with trickle feed instructions when the machine is at rest and ready for its next machining operation.
The Merry system cannot determine the location of the workpiece in relationship to the machine using the three interferometers alone. SOMaC is able to locate the machine relative to the workpiece using the single interferometer. Knowing this reference, SOMaC can provide delta correction commands to the machine controller after measuring the true position of the machine""s end effector to enhance the machine""s accuracy.
Real-time 3D optical measurement systems (e.g. laser trackers) are state-of-the-art measurement systems that obtain large quantities of accurate 3D data quickly. These optical measurement systems typically include an absolute ranging capability and a motorized angle steering head to steer the laser beam. The steering is controlled by a feedback system that continually drives the laser beam to follow (xe2x80x9ctrackxe2x80x9d) the retroreflector. The laser beam is directed from the laser tracker head into a retroreflective target which is mounted on the machine end effector. The return beam allows the tracking head to determine both the distance and the direction (i.e., the horizontal and vertical angles) to the retroreflector. These three measurements (range, horizontal angle, vertical angle) establish a spherical coordinate system which can easily be transformed into the Cartesian coordinate system.
Laser tracking systems have the following characteristics:
(1) Accurate 3D measurement of about 10 part per million (ppm) volumetric accuracy (0.1 mm in a 10 meter volume);
(2) Real-time measurement collection and transmission;
(3) Data rates, in excess of 500 3D measurements per second (and typically as high as 1000 measurements per second);
(4) Simple calibration;
(5) Virtually immune to errors caused by changes in air temperature and pressure when using a high quality compensator (refractometer); and
(6) Large measurement volume using a retroreflective target, typically a partial sphere up to 100 feet in diameter.
Absolute ranging tracking interferometers can reaquire a target that has been temporarily blocked. Absolute ranging tracking interferometers are highly desirable in manufacturing operations, because movement of the machines, parts, and operators in the factory can lead to beam breaks. We prefer to use absolute ranging tracking interferometers, but many of our applications can also use the interferometer systems that are less tolerant of beam breaks.
Laser trackers have been used in many applications such as measuring the digital contour of aircraft or automobiles, tooling inspections, and NC machine accuracy testing. The present invention currently uses laser trackers, but other optical or non-contact measurement systems can be substituted for these systems to provide the positional feedback for the system.
In the aerospace industry, gantry or post-mill drilling machines range in size up to 70 meters long. The largest of these machines have working volumes in excess of 700 cubic meters. The positioning tolerance requirements for these machines are typically less than 0.20 mm. Attaining 0.50 mm positioning uncertainty within a 100 cubic meter volume is difficult. To standardize the uncertainty statement for NC machines, it is common to state the uncertainty of the machine in parts per million (ppm). The uncertainty, multiplied by one million then divided by the longest diagonal distance in the machine volume is the capability in terms of parts per million (ppm). For example, a typical machine with a 0.5 mm positioning capability and 15 meter diagonal length would yield a capability of 33 ppm. Large volume drilling machine capability below 30 ppm is difficult to achieve. As manufacturers strive to improve part quality and reduce assembly costs, the demand for more accurate hole drilling has increased. In aerospace manufacturing, these tighter tolerances can be as small as 0.10 mm over a 15 meter diagonal, which yields a standardized requirement of 6.7 ppm. Such tolerances exceed the capability of most machines.
The present invention involves static optical machine control (SOMaC) and seeks to overcome the thermal and mechanical error sources inherent to large machines by using an absolute ranging laser tracking system or its equivalent to measure the position and orientation of the machine end effector when the machine is stationary. These measurements are reported automatically through the SOMaC computer through trickle feed instructions for position adjustment to the machine controller. The machine controller then corrects the machine position as required. SOMaC uses an iterative technique to control the accuracy of the NC machine end effector. A standard deviation control protocol eliminates the effect of xe2x80x9cnoisexe2x80x9d at the rest position. The protocol discriminates the rest position from machine motion or vibration. We incorporate alarms for tilt, especially for differential tilt of the machine, part, or tracker (using dual axis tilt sensors) and for temperature variation in the factory.
SOMaC uses xe2x80x9ctouch probexe2x80x9d or coordinate measurement machine software to locate critical features associated with the part during system calibration. These measurements establish a part frame of reference. During machining, SOMaC controls further operations based upon remeasurement and assessment of the location of these critical features. Because we establish a part frame of reference to which the machine adjusts, we eliminate the need for accurate part fixturing to establish a true position reference. The actual location of the part (and its features) is established by measuring the location of the features and comparing the measured location with the location established in a digital definition or digital dataset representation (CAD model) of the part. The comparison is used not only to calculate the actual part position, but also to calculate a xe2x80x9cscale factorxe2x80x9d for adjusting machine commands to compensate for differences between the actual part and the digital dataset representation. This xe2x80x9cautoscalexe2x80x9d feature, in effect, alters the NC Media derived from the engineering specification of the part to accommodate physical changes to the part that occur during machining, such as changes in the size of the part arising from changes in the factory temperature for the design standard 20xc2x0 C. (68xc2x0 F.). For example, we adjust the machine media to reflect the effect of expansion or contraction of the part because of its natural coefficient of thermal expansion. For xe2x80x9cautoscale,xe2x80x9d we determine in parallel whether the scale factor that we calculate is consistent with the changes in size we would expect from changes in the factory temperature. We monitor the factory temperature (but could also monitor the part temperature and machine temperature) and rescale at appropriate intervals (e.g., a change of 2xc2x0 or 5xc2x0 at a user defined alarm set point) when the temperature changes. xe2x80x9cAutoscalexe2x80x9d is a batch or interval adjustment rather than a continuous rescaling, which reduces the processing required.
SOMaC preferably involves accurately positioning the end position of the end effector of a static machine with an independent 3D optical measurement device. It is applicable to any machine in which the positioning accuracy of the measurement device is better than the machine accuracy, which is usually true for laser trackers and large machines that have at least one axis greater than fifteen feet. By controlling the position of the end effector through the machine controller indirectly with the independent optical measurement system, the thermal errors and misalignment errors in the framework of the machine are rendered innocuous because true position of the end effector is monitored and adjusted without regard to these sources of error. With the SOMaC system using xe2x80x9cbest machiningxe2x80x9d practices, we obtain a maximum linear true position error of about 0.003 inch (i.e., 0.0015 inch radial misplacement) in a ten foot volume with a much tighter distribution for the offset error than is achievable simply with the machine tool""s standard controller. We direct the end effector closer to the desired location specified in the digital dataset that defines the part or assembly using the machine tool""s controller. Then, we verify that the end effector is actually in the correct location using an independent, highly accurate laser tracker or other position sensor. If out of position, we adjust the position of the end effector by sending a delta adjustment to the machine controller.
While some sources of error may be nonlinear to cause SOMaC to lose accuracy, we use least squares fit algorithms (or other appropriate regression analysis) to minimize these nonlinearities. Our first order (linear) correction is fairly robust and achieves a significant improvement in accuracy. SOMaC can accommodate more sophisticated algorithms as nonlinearities and anisotropies are understood.
SOMaC uses feedback from an optical measurement device and associated software to trickle feed position corrections to an existing machine encoder to improve machine accuracy. The system is fast, inexpensive, and reliable to provide position accuracy that is independent from the repeatability of the machine or the relationship of the machine to the workpiece. The system provides absolute spatial orientation/position information. Our preferred system includes the following features:
A. SOMaC controls the machine position at the end effector, thus eliminating major contributors to overall machine inaccuracy.
B. SOMaC can be used on a probe-capable machine to transfigure the machine into an accurate Coordinate Measuring Machine (CMM).
C. SOMaC transforms tracker measurements into the part""s coordinate system, which reduces the complexity of the part-machine alignment calibration process.
D. SOMaC provides a Graphical User Interface (GUI) which allows the user to control various aspects of the machining operations. The software is a xe2x80x9creal-time, event drivenxe2x80x9d system that interprets text files for the configuration and programming information.
E. SOMaC provides a graphical user interface displaying:
(i.) the positioning accuracy desired;
(ii.) statistical parameters relating to tracker measurement accuracy;
(iii.) timing and position thresholds;
(iv.) operational modes;
(v.) offset and tracker/machine alignment;
(vi.) NC feed control;
(vii.) tracker position display and sample rates;
(viii.) a temperature monitor and tilt monitor alarm set points,
(ix.) on-line help.
F. By the nature of its software architecture, SOMaC is easily adapted to new machine controllers. An encoder interface software module is the only change needed to adapt the system to a new encoder/machine controller.
G. Portability. The trackers and workstation are physically portable and, therefore, a single system can be used to service many different machines.
H. Beam break recovery. SOMaC has two modes of recovery if a laser beam is interrupted.
(i.) Manual Recovery: the system halts and allows the operator to return the retroreflector manually to the tracker, regain beam-lock, and then continue.
(ii.) Automatic Recovery: the system returns the machine to a known location, commands the tracker to establish beam-lock, and then continues with the NC program.
I. SOMaC""s architecture is easily adapted to new optical measurement systems, multiple measurement systems, or hybrid measurement systems.
J. SOMaC uses xe2x80x9ctrickle-feedxe2x80x9d communication with a controller to integrate an NC machine with both the laser and the external software controller to create an easily packaged system that is capable of improving the accuracy of a machine. This method makes SOMaC applicable to a wide number of controllers with minimal integration effort.
K. SOMaC produces an audit trail of machining events. That is, SOMaC records the correction instructions it provides to the machine controller during the sequence of operations. With this data, it is easier to detect progressive machine drift or wear degradation or even to identify errors in the digital representation of the part.
L. SOMaC integrates the laser tracker with the machine in a computer remote from the machine controller so the system can be retrofit to many different NC controllers without software modifications to the controller.
The present invention relates to a method for improving the accuracy of machines. Machine mispositioning is corrected by providing delta position correction commands in machine media (e.g., NC Media) to a machine controller if a comparison of the true position of the machine tool under the control of the machine controller and the position in which the machine controller locates the machine tool based upon machine media instructions derived from an engineering specification of the part exceeds a predetermined offset threshold.
In one aspect, then, the present invention is a method for improving the accuracy of machines, comprising the steps of: (a) driving a machine tool having an end effector to a first commanded location based upon commands generated from a digital definition of the part or assembly on which the machine tool works; (b) precisely measuring the position of the end effector when the machine tool stops at the first commanded location; (c) comparing the measured position with the first commanded location; (d) sending delta correction commands to the machine tool to adjust the position of the end effector if the difference between the measured position and commanded position exceeds a predetermined threshold; (e) optionally, scaling the commanded position for thermal effects as derived from the digital definition with a thermal effect scale based upon deviation of the actual temperature of the workspace from the theoretical design criteria and adjusting the delta correction command in response to the thermal effect scale; (f) optionally, scaling the commanded position as derived from the digital definition with a configuration scale based upon measurement of a change in location of critical features on the part, assembly, or associated tooling and adjusting the delta correction command in response to the configuration scale; and (g) optionally, measuring the machined part using an inspection probe mounted in the machine and guided to a machine commanded position with machine commands derived from a digital definition of the part, the inspection involving measuring a set of inspection features, the measuring being conducted to accept the part and being augmented by providing delta correction commands to the machine to increase its accuracy, the delta correction commands being derived from comparing measurements of true position of the probe with the machine commanded position.
The invention also relates to a method for accepting a product by measuring its features in inspection tooling, comprising the steps of: (a) positioning a measurement probe in a spindle of a machine; (b) measuring selected inspection features on the product as a set of inspection measurements with the probe in accordance with an inspection sequence derived from the intended configuration of the product as specified in a digital definition of the product; and (c) scaling the intended configuration of the product as specified in the digital definition to adjust the relative size and position of features in accordance with measurement of changes in the actual configuration of the product in the inspection tooling caused by changes in factory conditions. Generally, such acceptance is done before removing the product from manufacturing tooling and manufacturing machines associated with making the product. This product acceptance method allows a manufacturer to use a machine tool for product inspection rather than needing a precision Coordinate Measuring Machine. Such xe2x80x9cinspectionxe2x80x9d permits greater use of machine tools and reduces the overall capital expenses for tooling by making machine tools versatile as inspection devices.
The present invention also relates to computer software storage media having computer-readable information recorded to provide repositioning commands to a machine controller based upon a comparison of the measured true position of a machine tool end effector or inspection probe positioned at a commanded position using positioning data derived from a dataset representation (i.e., digital definition) of a part with the position to which the machine controller moves the end effector following the machine media implementing the position data.
The present invention also relates to a machine tool system having improved positioning accuracy, comprising: (a) a machine tool, including an end effector, adapted for performing a machining operation of a part; (b) a machine controller coupled with the machine tool for commanding movement of the machine tool to a commanded position through position control media derived from an engineering drawing or a digital dataset representation of the part; (c) at least one laser tracker positioned for measuring the true position of the end effector; (d) a computing system for comparing the measured position of the end effector with the commanded position and for providing trickle feed adjustment signals to the machine controller to offset any difference between the commanded position and the measured position; and (e) optionally, means for adjusting the commanded position derived from the digital dataset representation of the part for time varying factory conditions that impact size or orientation of the part.
The present invention also relates to a method for modifying the spatial specification of machine media representing a part configuration to compensate for a temperature difference between the design temperature and the actual temperature of the part or a manufacturing workcell, comprising the steps of: (a) creating a computer-readable dataset representation of an intended configuration of a part at a reference temperature; (b) upon a temperature change of a predetermined interval, measuring the part in the manufacturing workcell in sufficient locations to identify the relative change in size or orientation of the part attributable to factory conditions; and (c) adjusting the dataset representation by the ratio of the remeasurement/reference measurement.
Finally, the present invention relates to a method for modifying the spatial specification of machine media representing a part configuration to compensate for a changes in a part during its manufacture in a factory, comprising the steps of: (a) measuring the part in sufficient locations to identify the actual configuration in a first digital representation; (b) remeasuring the part to produce a second digital representation of the part; (c) comparing the second digital representation with the first digital representation to determine a scale factor; and (d) adjusting the machine media in accordance with the scale factor.